Gas turbine engines include a compressor, a combustor and a turbine arranged in flow series and generally about a rotational axis. During operation, the compressor supplies compressed air to the combustor and which is mixed with a gaseous or liquid fuel. The air/fuel mixture is then burned and the combustion gas is channelled via a transition duct to the turbine section. The combustion gases force rotation of the turbine which in turn drives the compressor via an interconnecting shaft. For gas turbine engines having a cannular combustor arrangement, which is an annular array of combustor cans each having at least one a burner and a combustion chamber, the transition duct has typically a circular inlet that interfaces with the combustor chamber and an outlet in the form of an annular segment. An annular array of transition duct outlets form an annulus for channelling the combustion gases to the turbine.
The transition duct is manufactured from sheet metal walls or could be cast having relatively large surface areas. These large surfaces incur significant thermal expansions and contractions which cause stresses within the walls. These thermal stresses are increased where there is a significant thermal gradient. In addition, the inherent geometry of the transition duct, which transitions from a circular inlet to an annular segment and the interfaces between the combustor and turbine, creates unique stress regimes when subjected to the hot working gases from the combustor.
The service life of the transition duct is partly determined by the absolute temperature it experiences and the temperature distribution or gradient across the component. To protect the component material from over-heating, conventionally thermal-barrier coatings (TBC) are applied on the hot gas exposed area and often the entire hot side of the part, which in this case is the internal surface of the transition duct. Here the temperature distribution of the combustion gas flow egressing the combustor is not uniform and therefore the application and uniform thickness of TBC is determined by the maximum temperature experienced and the material's thermal capability. The temperature difference across the transition duct's internal surface can be in the region of 700° C. TBC is applied to the entire internal surface of the transition duct because failure of the TBC commonly occurs at the edges or discontinuities of the TBC. Failure is usually a debonding of the TBC material from the surface of the transition duct or cracking of the TBC. For this reason, conventional application of TBC is made to the entire internal surface of the transition duct which includes an inlet ring and an exit flange that form a complex geometrical shape.
Conventional application of TBC is in a uniform thickness and to the entire internal surface of the transition duct and which is satisfactory in reducing the temperature experienced by the sheet metal wall material. However, there remains a considerable thermal gradient across the transition duct and in-service experience has uncovered thermally-induced debonding and cracking of the TBC because of thermal stresses.